We are developing a novel immunomodulatory approach to arrest pancreatic islet injury and restore specific immune tolerance in new-onset Type 1 diabetes mellitus (T1D). Damage of pancreatic islets in T1D may result from excess pathogenic Th1/Th17 cells and deficient tolerance-inducing antigen (Ag)-specific Treg (FOXp3+CD25+) and Tr1 CD4+ cells (FOXp3-CD25- IL-10high). Lacking suitable levels of Tr1 cells, pathogenic activated Th1/Th17 cells are unopposed, resulting in islet cell damage and destruction. Our approach, which redirects the polarization of autoimmune-activated Th1/Th17 cells towards tolerance-inducing Tr1 cells, is grounded in research showing that the transfusion of ex vivo produced Ag-specific Tr1 cells is protective in experimental models of autoimmunity, such as experimental T1D and islet cell transplantation. Ex vivo repolarization of Th1 cells, however, is cumbersome and expensive. In our in vivo approach, we parentally administer CXCL11 (mR-412), a CXCR3-binding ligand that we have recently identified as a counter-regulatory chemokine. Therapy with a long-acting Fc fusion protein of CXCL11 (mR-411) profoundly suppresses murine experimental allergic encephalomyelitis (EAE), even when initiated after disease onset. Moreover, Ag-specific effector Th1 cells isolated from EAE donors treated in vivo with mR-411 redirect their polarization into Tr1 cells and suppress EAE in adoptive transfer experiments. In contrast to pan-suppressive immunomodulators, mR-411 induces tolerance that is Ag-specific for the active disease yet preserves generalized immunity to previously encountered antigens. We now seek to extend these observations and establish proof-of-concept that mR-412 establishes immune tolerance of islets in a murine model of T1D. Specific Aim #1: Establish the potency, dose-dependence, and durability of mR-412 rescue therapy of well- established diabetes mellitus in a spontaneous murine T1D model. mR-412 (0, 4, 40 ?g/kg IP 2 X per week) will be dosed for 18 weeks to female diabetic NOD mice beginning at 180 days of age, a time point characterized by established hyperglycemia. Mice will be terminated either directly at the cessation of mR-412 therapy (acute group) or 12 weeks thereafter (recovery group). We will examine pancreases for histologic and immunohistochemical evidence of islet injury and determine insulin content. Spleen and pancreases will be examined for: 1) intracellular cytokine expression in lymphocytes, and 2) immunohistochemical identification of T cell subset infiltration and the phosphorylation of T-bet and STAT3. Plasma concentrations of mR-412 will be determined by ELISA and compared to functional outcomes. We expect that mR-412 will restore normoglycemia until the conclusion of the recovery period. Supportive data will include dose-dependent demonstration that mR-412 produces favorable biochemical and immunologic effects on islets harvested at the time of sacrifice, including the maintenance of islet insulin content, the blockade of T-cell infiltration nd T-bet phosphorylation, and STAT3 phosphorylation.